Initially not appreciating Robert, powerful guy (5)
I believe the answer is:
nabob
'powerful guy' is the definition.
(nabob can mean a rich or powerful person)
'initially not appreciating robert' is the wordplay.
'initially' suggests taking the first letters.
'robert' becomes 'bob' (short for Robert).
The initial letters of 'not appreciating' is 'na'.
'na'+'bob'='NABOB'
(Other definitions for nabob that I've seen before include "he may be loaded" , "moneybags" , "Powerful man" , "Wealthy influential person" , "Person of great wealth" .)